This proposal focuses on the characterization of a novel protein that is encoded by PKDH1, the gene responsible for causing autosomal recessive polycystic kidney disease (ARPKD), by the generation of mouse lines with a targeted mutation in PKDH1 and by the production of poly- and monoclonal antibodies against the gene product. Recently, we identified a novel gene, named PKHDl-tentative (PKHD1-T), whose expression can mainly be detected in the kidney, pancreas, and liver 8. This gene encodes a 3396-amino-acid protein, which was named tigmin. While we were validating this gene as PKHD1, two groups reported their own identification of PKHD1 9,10 They showed this gene to encode a novel protein they called fibrocystin or polyductin, respectively, which encoded 4074 amino acids. Both fibrocystin and polyductin share an almost identical amino acid sequence with tigmin (except at the C-terminus), indicating that they are encoded by the same gene. To understand the functional roles of PKHD1, we will use molecular, cell biological, and transgenic approaches to address the cellular physiology of tubulogenesis and tubular maturation in affected tissues such as the kidney and liver. We will also address the mechanisms by which the deficiency of PKHD1 causes cyst formation and/or fibrosis. The generation of antibodies against this novel protein will provide a powerful tool for revealing the cellular and subcellular localization of PKHD1 and assessing its biological features during kidney and liver development. In addition, the establishment of mouse models by genetically mutagenizing the murine homologue of human PKHD1 and by inserting into the gene the cDNAs for detectable reporters will enhance the characterization of its gene product and the dissection of the molecular pathogenesis of ARPKD. We will take advantage of mouse models with targeted mutations in Pkhdl, Pkdl, and Pkd2, which were or will be generated by us, as well as of existing mouse ARPKD-like models that have risen spontaneously and been created experimentally to dissect the genetic relationship between these gene products. Taken together, the results of these studies will significantly advance our understanding of the pathogenesis of polycystic kidney diseases and the molecular mechanism of cyst formation and growth.